Extracting i18n functionality to i18n core library (#16914)

* extracting i18n functionality to i18n core library

* Removing utils.T

* Adding documentation and changing one function name for better explanation

* Changing other missing utils.T

* Adding license string

* Renaming corelibs to pkg

* Renaming corelibs to pkg (moving directory)

* Renaming from pkg to shared

* Fixing bodyPage.Html casing

* Fixing merges

* Fixing merge problem

* Fixing tests

app/web_conn_test.go

@@ -10,7 +10,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/utils"
+	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 )
 
 func TestWebConnShouldSendEvent(t *testing.T) {
@@ -28,7 +28,7 @@ func TestWebConnShouldSendEvent(t *testing.T) {
 	basicUserWc := &WebConn{
 		App:    th.App,
 		UserId: th.BasicUser.Id,
-		T:      utils.T,
+		T:      i18n.T,
 	}
 
 	basicUserWc.SetSession(session)
@@ -47,7 +47,7 @@ func TestWebConnShouldSendEvent(t *testing.T) {
 	basicUser2Wc := &WebConn{
 		App:    th.App,
 		UserId: th.BasicUser2.Id,
-		T:      utils.T,
+		T:      i18n.T,
 	}
 
 	basicUser2Wc.SetSession(session2)
@@ -60,7 +60,7 @@ func TestWebConnShouldSendEvent(t *testing.T) {
 	adminUserWc := &WebConn{
 		App:    th.App,
 		UserId: th.SystemAdminUser.Id,
-		T:      utils.T,
+		T:      i18n.T,
 	}
 
 	adminUserWc.SetSession(session3)